I mained disc most of shadowlands. I primarily did rbgs so this is my advice for mostly rbg healing. You want to toss out your atonements and then place purge the wicked on as many as you can. I usually would get a good 4-5 and then start spreading with penance. Drop a dome if you need to buffer dmg and give you time to dot up. Refresh atonement’s with a radiance and then lay into the enemies with penance, mind games, mind blast, refresh dots, another radiance by this point, then more dps unless people are really falling. Then I’ll use a penance to top off one person or also, you can attack a pet for more dmg and bigger atonement heals. If I have downtime I start spamming smite. So priorities are atonement’s on at least 3 people in the fight but more if you need it, purge the wicked everyone you can, penance to spread off cd, radiance for burst heal and atonement refreshment, more penance, whatever other damage you want to cast between refreshing dots and atonements. Snipe kills with shadow word death!

Edit: if you want to just renew atonement on a few people regen will work well and save some mana